The long-term goal of my research is to understand the factors that guide retinal ganglion cell axons along the optic tract and which direct them to appropriate target regions. The specific aim of this grant is to understand (i) how the target (i.e., the superior colliculus) affects ganglion cells and growing optic axons and (ii) how diffusible factors derived from innervated target regions might be regulated. The overall hypothesis to be tested by these experiments is that diffusible factors released by the superior colliculus upon innervation by retinal axons influence the survival of and neurite outgrowth from retinal ganglion cells. These issues will be addressed with in vitro studies, with further studies using neural transplants and in studies which examine the expression of trophic factor genes in the superior colliculus during development. The first group of studies will examine the ability of explanted and transplanted target regions to influence optic axon outgrowth: retinal explants in collagen gel cultures, or on substrates coated with the cell-adhesion molecule L1, will be co-cultured with superior colliculus to test whether previously innervated targets influence optic axon outgrowth. A parallel study will examine whether optic axons in vivo are attracted to an ectopic target. A second group of studies will examine the effects target-derived factors on retinal ganglion cells: the first will use a retinal ganglion cell bioassay to test in vitro whether media conditioned by non-innervated superior colliculus (derived from pre-innervation normal mice or congenitally blind mice) lacks a protein that is present in the active fraction of innervated SC. The second study will use in situ hybridization techniques to correlate BDNF mRNA levels in the superior colliculus with optic axon input. Although the main goal of these studies is to address a basic developmental question (an understanding of how retinal ganglion cell axons interact with their target), these studies also have direct implications for studies on therapeutic efforts to form optic connections in damaged or deficient visual systems.